US-A 2009/0173166 A1 discloses a device for measuring fluid flow rates, comprising a flow channel and at least one mark depositor to introduce at least one mark, more specifically a thermal mark, in the flow channel at an introduction location, thereby causing a variation in the fluid. The device furthermore comprises a plurality of mark detection sensors, such as temperature sensors situated upstream and downstream of the mark depositor, configured for detecting the variation in the fluid caused by the introduction of the at least one mark.
A thermal marker is generated in a substantially discrete manner with respect to time in order to make it observable. However, due to thermal diffusion in the fluid and the flow channel, the thermal marker becomes less discrete over time. Given said thermal diffusion, the thermal marker loses its characteristics that enable the thermal marker to be clearly distinguishable. That is, the mark detection sensors only observe a gradual rather than an abrupt temperature increase when the thermal marker is passing by. Consequently, it is relatively difficult to determine the point in time at which the marker actually crosses the marker detection sensor, thereby causing significant inaccuracy. As a result, the device disclosed in US-A 2009/0173166 A1 is not capable of accurately measuring the velocity at which the fluid is flowing through the flow channel.